In general, as magnetic recording media for audio tapes, video tapes, computer tapes (disks, memory tapes), etc., magnetic recording medium each having on a non-magnetic support a magnetic layer composed of a ferromagnetic powder dispersed in a binder have been used.
Recently, high-density recording has been required for these magnetic recording media, and the increase of signal and decrease of noise in signal/noise have been attained by fining the particle sizes of the ferromagnetic powder, using a ferromagentic allow powder, increasing the filling extent of the ferromagentic powder in the magnetic layer, or greatly improving the smoothness of the magnetic recording medium.
However, for high-density recording, it has been required that the magnetic recording medium has good antistatic characteristics, good head cleaning characteristics, good running durability characteristics, and good head abrasive characteristics, and further that the occurrence of the drop out or decrease of signal/noise ratio in recording and reproducing caused by foreign matters such as dusts, etc., on the magnetic recording medium is less.
For the purposes, carbon black and abrasives having a Mohs' hardness of at least 8 are used. Practically, these techniques are disclosed in U.S. Pat. Nos. 3,630,910, 3,833,412, 4,614,685, 4,539,257, and JP-A-59-193533 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In regard to the magnetic layer of the magnetic recording medium produced by the aforesaid technique, it is considered that the magnetic layer is generally very smooth since the granular components are strongly fixed in the layer. However, according the inventor's investigation, it has been found that insufficiently fixed granular components (e.g., a ferromagnetic powder and an abrasive), etc., exist on the surface of the magnetic layer. Such insufficiently fixed granular components sometimes release in, e.g., a video tape, and attach to a magnetic head to cause clogging of the magnetic head and also to cause the occurrence of drop out. Also, such a release of the ferromagnetic powder gradually decreases the amount of the ferromagnetic powder near the surface of the magnetic layer and hence by repeating running of the magnetic recording tape, there also occurs deterioration in the electromagnetic properties (lowering of output).
The inventors previously found a process involving grinding of the magnetic layer surface of a magnetic recording medium for preventing the aforesaid occurrence of the drop out and clogging of the magnetic head and for reducing lowering of output by repeated running of the magnetic recording medium, and disclosed in JP-A-62-172532 and JP-A-63-98834. That is, in the aforesaid invention, by grinding the surface of a smoothened magnetic layer using a high hardness grinder such as a diamond wheel or a fixed sapphire blade, attached matters on the surface of the magnetic layer and granular components which are in an easily releasable state are removed to decrease the amount of releasing materials from the magnetic layer. By grinding the surface of the magnetic layer as described above, the occurrences of drop out and clogging of the magnetic head and the occurrence of lowering of the output by long time running of the magnetic recording tape can be effectively prevented.
Also, in the field of a magnetic disk, it is known to varnish for surface smoothing. For example, a surface smoothing apparatus for a magnetic disk base plate by continuously supplying a wrapping film onto the sliding surface of the magnetic head (JP-B-58-46768, the term "JP-B" as used herein means an "examined published Japanese patent application") and a surface smoothing method of a base plate for a magnetic disk by rotating a base plate for a magnetic disk and contacting a varnish head having a roughened portion composed of a grinding grain layer containing hard fine grains formed on the slider surface thereof to the surface of the rotating base plate (JP B-58-46767) are known.
Other techniques relating to varnishing of magnetic recording media are also disclosed in JP-A-56-90429 and JP-B-63-259830.
However, even by these techniques, it is very difficult to balance the running durability characteristics, head abrasion characteristics, and head cleaning characteristics, and the reduction of lowering of drop out is yet insufficient.
Practically, if the magnetic recording medium is too smooth, the coefficient of friction is increased to deteriorate the running durability characteristics. Thus, carbon black and an abrasive having a Mohs' hardness of at least 6 are used for a magnetic recording medium for securing the running durability thereof but the abrasion of the magnetic head is increased. Also, if the addition amount of the abrasion in the aforesaid additives is decreased for adjusting the head abrasive characteristics, the head cleaning characteristics are deteriorated.
Thus, it is very difficult to adjust the aforesaid characteristics in good balance.